Method of spheroidizing steel by rapid heating



United States Patent O METHOD OF SPHEROIDIZING STEEL BY RAPID HEATING Charles A. Turner, Jr., Flourtown, vPa, assignor to Selas Corporation of America, Dresher, Pa., a corporation of Pennsylvania No Drawing. Filed Apr. 24, 1958, Ser. No. 730,544

3 Claims. (Cl. 148-134) The present invention relates to the heat treatment of carbon and low alloy steels, and more particularly to a process of spheroidizing these steels by short-cycle heatmg.

This spheroidizing process has been called, variously, process, sub-critical or spheroidization annealing. It is basically a softening treatment, applied to increase the ductility of high carbon steels that are to be cold formed, or, in the case of tool steels, to improve machinability. For some cold forming operations low and medium carbon steels are similary treated.

A brief description of the micro-constituents of steel and the influence of heat upon them will serve to clarify the differences between past practice and the practice of this invention.

The microstructure of any steel is composed of ferrite which is soft and carbides which are hard. The hardness, or strength, depends upon the average distance between carbides. This distance is greatest and, hence, the steel softest when the carbides are in a coarse globular form. This spheroidization proceeds by the diffusion and assuming of a globular form by the carbides in the ferrite matrix, and the rate of this process increases with increasing temperature.

If the steel is heated to the Ac lower critical temperature (approximately 1330 F.), however, ferrite is converted from alpha to gamma form or austenite. This austenite is capable of absorbing or dissolving the carbides, and this solution is completed at the AC3, or upper critical temperature, which varies with the carbon content. Thus, if the carbon is dissolved by heating, its transformation to spheroidal form will require very slow cooling or prolonged holding during the cooling cycle at a temperature just below the lower critical.

Generally, and by usual practice, a spheroidization treatment is applied to hot-rolled steels; that is, steels which have been air-cooled from the finish hot-working temperature. In such steels, the carbides exist in a platelike, or lamellar form, called pearlite. If this pearlitic steel is heated to a temperature just under the lower critical and held for many hours, the plate like carbides will gradually change to a spheroidal or globular form, due to a surface tension phenomenon.

Other conventional spheroidizing treatments employ heating to a temperature just above the upper critical and cooling very slowly (about F. per hour) through the critical range, or heating to a temperature in the critical range (between Ac, and Ac and cooling slowly. A variation of these treatments aiming to decrease the overall time cycle involves alternately heating to just above and cooling to just below the lower critical temperature. The final step, however, consists of holding at a temperature just below the lower critical.

All these spheroidization annealing cycles are time consuming. In addition, they are always performed as a batch treatment with a heavy loading of multiple pieces, or coils in the case of wire and rod. Dilferent portions Patented May 31, 1960 of the charge heat and cool atvarying rates, such that it is impossible to develop a uniform structure.

The object of the present invention is to provide a rapid method of spheroidizing steel so that the treatment can be used in a production line between drawing operations, or between cold drawing to size and cold heading for bolts and the like.

In following the invention, hot rolled steel is heated rapidly and continuously to a temperature somewhat above the Ac critical and permitted to cool in air. This provides the advantage of a uniform heating and cooling of each part of the work, thereby producing uniformity of annealed physical properties. During this process a majority, if not all, of the carbides will assume the characteristic spherical shape. Cold working prior to the treatment appears to provide a triggering effect for accelerating or speeding up the formation of the spherical carbide particles.

In practicing the invention, it is preferred to move the wire, or other shape of steel, continuously through a furnace of a given temperature at a rate sufiicient to permit the steel to be brought up to the desired temperature at the moment it leaves the furnace. The temperature of the furnace is usually at a minimum of 2000 F., but is always high enough so that the work is heated to a temperature of from F. to 300 F. above the Ac; critical temperature before the carbides have time to dissolve. The work is heated as fast as possible, and usually at a rate no slower than two minutes per inch. Chromium steels should be heated about 100 F. higher than carbon steels since the carbides have more sluggish diffusion rates. After being heated, the metal is immedi ately air cooled. During this process the carbides assume a globular shape, with this formation being accelerated by the high temperatures employed. Due to the rapidity of heating and cooling, there is insufficient time for the carbides to be dissolved in the gamma iron. Thus, upon cooling it is not necessary for the carbides to precipitate before they can begin to assume a globular form. Cold working of the steel prior to this short cycle treatment is beneficial in that it introduces microstresses which appear to have a triggering eifect upon the carbides. This accelerates their response to the formation of spheroidal shape under heat.

As a specific example of the process, 0.229" diameter wire of A181 1035 steel was conveyed continuously through a heating chamber 72" long operating at 2400 F. The wire traveling at a speed of thirty feet per minute was evenly and uniformly heated to 1500 F., or F. above the lower critical, in 12 seconds, followed by air cooling. The rapid heat transfer to the work results from the high temperature gradiant between the furnace and the wire.

The dwell time at temperature above the lower critical in this case, is so short that no carbides are dissolved in the austenite, but diffuse and change their shape to the globular form rapidly under the accelerating influence of the high heat.

While in accordance with the provisions of the statutes, I have described the best form of embodiment of my in vention now known to me, it will be apparent to those skilled in the art that changes may be made without departing from the spirit and scope of the invention set forth in the appended claims, and that in some cases certain features of my invention may be used to advantage without a corresponding use of other features.

What is claimed is:

1. The method of spheroidizing carbon and low alloy steel which comprises moving the steel continuously through a furnace at a temperature sufficiently above the temperature of the steel to raise the steel temperature from 100 F: to BO O F. above the lower critical temperature at a rate fast enough that the carbides in the steel do not have time to dissolve, and immediately upon the attainment of such temperature air cooling the steel. 2. The method of spheroidize annealing carbon or low alloy steel which comprises moving the steel continuously through a furnace having a temperature of at least 2000 F. at a rate sufiicient to raise the temperature of the steel to 1500 F. at the exit of the furnace before the carbides have dissolved, and air cooling the steel. r

'3. The method of spheroidize annealing carbon or low egesasao alloy steel which comprises placing the steel in a furnace at an elevated temperature sufiiciently highto raise the steel to a temperature from 100 F. to 300 F. above its lower critical temperature fast enough so that the car- References Cited in the file of this patent The Iron Age, Nov. 20, 1952, volume 170, pages 140-444, by Turner et al. 

1. THE METHOD OF SPHEROIDIZING CARBON AND LOW ALLOY STEEL WHICH COMPRISES MOVING THE STEEL CONTINUOUSLY THROUGH A FURNACE AT A TEMPERATURE SUFFICIENTLY ABOVE THE TEMPERATURE OF THE STEEL TO RAISE THE STEEL TEMPERATURE FROM 100*F. TO 300*F. ABOVE THE LOWER CRITICAL TEMPERTURE AT A RATE FAST ENOUGH THAT THE CARBIDES IN THE STEEL DO NOT HAVE TIME TO DISSOLVE, AND IMMEDIATELY UPON THE ATTAINMENT OF SUCH TEMPERATURE AIR COOLING THE STEEL. 